The present invention relates to a semiconductor light-emitting device which provides improved noise characteristics when operating at low output powers, and to a method for fabricating the light-emitting device, and a method for driving the same.
FIG. 14 shows a prior-art refractive index guided semiconductor laser device disclosed in Japanese Journal of Applied Physics by A. Kuramata et al., 37(1998), L1373.
For example, as shown in FIG. 14, on a substrate 101 of sapphire, formed are the following layers, each of which is formed of a III-V compound semiconductor. That is, grown by crystallization on the substrate 101 are an n-type semiconductor portion 102 containing an n-type contact layer, an active layer 103, and a p-type semiconductor portion 104 containing a p-type contact layer.
The upper portion of the p-type contact layer in the p-type semiconductor portion 104 has a ridge portion patterned in the shape of a stripe, where a p-side electrode 105 is formed on the entire surface of the ridge portion. In this structure, a region of the active layer 103 underlying the p-side electrode 105 acts as a cavity in which lasing takes place.
The n-type contact layer of the n-type semiconductor portion 102 is exposed on one side region of the p-side electrode 105, where an n-side electrode 106 is formed substantially on the entire surface of the exposed surface.
A forward drive current is applied from the p-side electrode 105 to the n-side electrode 106. When the drive current has exceeded a predetermined lasing current threshold, a laser beam is launched from one facet of the active layer 103.
Suppose that a semiconductor laser device like the one shown in FIG. 14 is used to perform a write operation on an optical disc such as a high-density digital versatile (or video) disc (HD-DVD). To use a purple laser beam in this operation, it is necessary to deliver an output of 30 mW or more. In contrast to this, it is necessary to make the output of the purple laser beam as low as 1 mW for the read operation.
However, in the read operation, there is a problem that the prior-art semiconductor laser device causes the relative intensity of noise to increase as the output decreases even when a high frequency is superimposed on the drive current. This is because a current approximately equal in magnitude to the lasing current threshold is injected to allow lasing to take place, thereby causing the relative intensity of noise to increase due to the effect of relaxation oscillation in the lasing.
In addition, lasing at approximately the same injected current as the lasing current threshold causes a characteristic of the single mode to be degraded and multi-mode components to develop, thereby increasing the relative intensity of noise.
To reduce the relative intensity of noise, it is necessary to increase the frequency of relaxation oscillation. As one of the methods that are applicable to the reduction, it is conceivable to increase the differential gain. To increase the differential gain of lasing, an optical absorption region may be formed to increase the lasing threshold.
Alternatively, the slope efficiency (differential efficiency) may be reduced to increase the current required to deliver a lasing output of approximately 1 mW, thereby setting the operating current to a value greater than the lasing threshold.
Incidentally, the facets of a cavity could be increased in reflectivity to reduce the noise of the semiconductor laser device. However, this would cause the output (optical output) of the laser beam to be reduced as well. As described above, lased light of a high output power is required for the HD-DVD device to carry out the write operation. Accordingly, this makes it impossible to employ the means for increasing the reflectivity of the facets, which leads to a reduction in optical output.
On the other hand, to allow the semiconductor laser device to provide self-pulsation, it is necessary to provide a semiconductor optical absorption layer in or near the active layer 103.
However, this raises a problem that such an optical absorption layer provided in the semiconductor laser device itself would make it difficult to provide a high output power.
It is therefore an object of the present invention to provide a semiconductor light-emitting device which solves the aforementioned prior-art problems and provides a reduced relative intensity of noise even when operating at low output powers.
To achieve the aforementioned object, the present invention provides a semiconductor laser device which is provided with the p-side or n-side electrode divided to apply a drive current to only part of a divided electrode when the laser device is required to operate at low output powers for the read operation.
More specifically, a semiconductor light-emitting device according to the present invention comprises a first semiconductor layer of a first conductivity type formed substantially in a uniform thickness on a substrate and a second semiconductor layer of a second conductivity type formed in a uniform thickness on the first semiconductor layer. The light-emitting device also comprises an active layer, formed in a uniform thickness between the first semiconductor layer and the second semiconductor layer, for generating emission light. The light-emitting device further comprises a first electrode for supplying a drive current to the first semiconductor layer and a second electrode for supplying a drive current to the second semiconductor layer. The light-emitting device is adapted such that the first electrode or the second electrode is a divided electrode comprising a plurality of conductive members spaced apart from each other.
According to the semiconductor light-emitting device of the present invention, a drive current is applied to all the divided electrodes for the operation at high output powers. On the other hand, for the operation at low output powers, a drive current is applied to part of the divided electrodes to inject the drive current nonuniformly into the active layer, thereby forming an optical absorption region in the active layer. This causes the lasing current threshold to increase and the differential gain of lasing to thereby increase, thus making it possible to reduce the relative intensity of noise during the operation at low output powers.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is provided on a principal surface, having said active layer formed thereon, of the substrate.
In the semiconductor light-emitting device according to the present invention, it is preferable that the second electrode has a stripe pattern for forming a cavity in the active layer, and the divided electrodes are adapted to sit on the sides of an emitting facet and a reflecting facet of the cavity, respectively.
In the semiconductor light-emitting device according to the present invention, it is preferable that the first electrode and the second electrode are formed on the principal surface, having the active layer formed thereon, of the substrate.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is a p-side electrode for injecting holes into the active layer.
In this case, it is preferable that the p-side electrode has a stripe pattern formed on the second semiconductor layer, and the plurality of conductive members of the p-side electrode are spaced apart approximately 10 xcexcm or less from each other.
In the semiconductor light-emitting device according to the present invention, it is preferable that the divided electrode is an n-side electrode for injecting electrons into the active layer.
In this case, it is preferable that the n-side electrode is formed on a region of the first semiconductor layer, the region being exposed on one side of the p-side electrode, and the plurality of conductive members of the n-side electrode are spaced apart approximately 5 xcexcm or more from each other.
In this case, it is also preferable that the second electrode is a p-side electrode having a stripe pattern formed on the second semiconductor layer and the n-side electrode comprises first and second electrode portions formed on a region of the first semiconductor layer, the region being exposed on one side of the p-side electrode. It is also preferable that a dividing region disposed between the first and second electrode portions is formed at an angle of inclination greater than 0xc2x0 and less than 90xc2x0 to a direction, within the plane of the substrate, perpendicular to the longitudinal direction of the p-side electrode.
In this case, it is also preferable that the second electrode is a p-side electrode having a stripe pattern formed on the second semiconductor layer and the first semiconductor layer is exposed on both sides of the p-side electrode. It is also preferable that the n-side electrode comprises a first electrode portion formed on a region of the first semiconductor layer on one side of the p-side electrode and a second electrode portion formed on a region of the-first semiconductor layer on the other side of the p-side electrode. It is further preferable that the first and second electrode portions are formed so as to be asymmetrical in their planar shapes with respect to the p-side electrode.
In the semiconductor light-emitting device according to the present invention, it is preferable that the substrate is conductive. It is also preferable that the divided electrode comprises a first electrode portion provided on the surface opposite to the principal surface of the substrate having the active layer formed thereon and a second electrode portion provided on the principal surface of the substrate having the active layer formed thereon.
In this case, it is preferable that the divided electrode is an n-side electrode and the first electrode portion is provided substantially on the entire surface opposite to the principal surface of the substrate. It is further preferable that the second electrode portion is provided on part of the region on a side of the p-side electrode, and the second electrode is a p-side electrode having a stripe pattern formed on the second semiconductor layer.
In the semiconductor light-emitting device according to the present invention, it is preferable that the active layer is formed of a compound semiconductor having a composition of nitrogen.
Furthermore, it is also preferable that the active layer is formed of a compound semiconductor having a composition of phosphor.
A method for fabricating a first semiconductor light-emitting device according to the present invention comprises the steps of:
growing successively a first semiconductor layer of a first conductivity type, an active layer, and a second semiconductor layer of a second conductivity type, each substantially in a uniform thickness;
exposing part of the first semiconductor layer to form then a first electrode on the first exposed semiconductor layer;
forming a second electrode on the second semiconductor layer; and
forming a divided electrode by isolating the first electrode or the second electrode electrically into a plurality of electrodes.
The method for fabricating the first semiconductor light-emitting device comprises the steps of forming the first electrode for supplying a drive current to the first semiconductor layer and the second electrode for supplying a drive current to the second semiconductor layer, and thereafter forming a divided electrode by electrically isolating the first or second electrode into a plurality of electrodes. This ensures it to provide the semiconductor light-emitting device of the present invention.
In the method for fabricating the first semiconductor light-emitting device, it is preferable that the step of forming a divided electrode employs an etching method.
In the method for fabricating the first semiconductor light-emitting device, it is also preferable that the step of forming a divided electrode employs a lift-off method.
A method for fabricating a second semiconductor light-emitting device according to the present invention comprises the steps of:
growing successively a first semiconductor layer of a first conductivity type, an active layer, and a second semiconductor layer of a second conductivity type, each substantially in a uniform thickness;
exposing part of the first semiconductor layer to form then a first n-side electrode on the first exposed semiconductor layer;
forming a second n-side electrode on a surface opposite to a surface of the substrate having the active layer; and
forming a p-side electrode on the second semiconductor layer.
According to the method for fabricating the second semiconductor light-emitting device, the n-side electrode is a divided electrode since the first n-side electrode is formed on the first semiconductor layer and the second n-side electrode is formed on the surface of the substrate opposite to the active layer. This ensures it to provide the semiconductor light-emitting device of the present invention.
In the method for fabricating the first or second semiconductor light-emitting device, it is preferable that the active layer is formed of a compound semiconductor having a composition of nitrogen.
In the method for fabricating the first or second semiconductor light-emitting device, it is preferable that the active layer is formed of a compound semiconductor having a composition of phosphor.
The present invention provides a method for driving a first semiconductor light-emitting device. The first semiconductor light-emitting device comprises a first semiconductor layer of a first conductivity type formed on a substrate and a second semiconductor layer of a second conductivity type formed on the first semiconductor layer. The light-emitting device also comprises an active layer, formed between the first semiconductor layer and the second semiconductor layer, for generating emission light. The light-emitting device further comprises a first electrode for supplying a drive current to the first semiconductor layer and a second electrode, having the shape of a stripe, for supplying a drive current to the second semiconductor layer. The first electrode or the second electrode is a divided electrode divided in the longitudinal direction of the second electrode, and the divided electrode comprises a first electrode portion located on the side of an emitting facet and a second electrode portion located on the side of a reflecting facet. The method comprises the step of applying a first drive current to the first electrode portion and the second electrode portion to increase a relative lasing output of a laser beam. The method also comprises the step of, to decrease a relative lasing output of a laser beam, applying the first drive current to the first electrode portion as well as applying a second drive current to the second electrode portion, the second drive current being less in magnitude than the first drive current, or applying no second drive current to the second electrode portion. Alternatively, the method comprises the step of, to decrease a relative lasing output of a laser beam, applying the first drive current to the second electrode portion as well as applying the second drive current to the first electrode portion, the second drive current being less in magnitude than the first drive current, or applying no second drive current to the second electrode portion.
According to the method for driving the first semiconductor light-emitting device, a current can be injected nonuniformly into the active layer for the operation at low output powers, thereby forming an optical absorption region in the active layer. This causes the lasing current threshold to increase and the differential gain of lasing to thereby increase, thus making it possible to reduce the relative intensity of noise during the operation at low output powers.
The present invention provides a method for driving a second semiconductor light-emitting device, the semiconductor light-emitting device comprising: a first semiconductor layer of a first conductivity type formed on a substrate, a second semiconductor layer of a second conductivity type formed on the first semiconductor layer, an active layer, formed between the first semiconductor layer and the second semiconductor layer, for generating emission light, a first electrode for supplying a drive current to the first semiconductor layer, and a second electrode, having the shape of a stripe, for supplying a drive current to the second semiconductor layer, wherein the first electrode or the second electrode is a divided electrode divided at an obverse surface and a reverse surfaces of the substrate, and the divided electrode comprising a first electrode portion which covers almost all a surface of the substrate opposite to a surface of the active layer and a second electrode portion located on the emitting face side or the reflecting facet side on the first semiconductor layer, the method comprising the steps of: to increase a relative lasing output of a laser beam, applying a first drive current to the first electrode portion, and to decrease a relative lasing output of a laser beam, applying the first drive current to the second electrode portion, and applying a second drive current to the first electrode portion or applying no second drive current to the second electrode portion, the second drive current being less in magnitude than the first drive current.
According to the method for driving the second semiconductor light-emitting device, a current can be injected nonuniformly into the active layer for the operation at low output powers, thereby forming an optical absorption region in the active layer. This causes the lasing current threshold to increase and the differential gain of lasing to thereby increase, thus making it possible to reduce the relative intensity of noise during the operation at low output powers.
In the method for driving the first or second semiconductor light-emitting device, it is preferable that the first drive current is allowed through resistance varying means to generate the second drive current.
In the method for driving the first or second semiconductor light-emitting device, it is preferable that the peak value of the second drive current is approximately one half the peak value of the first drive current.
The present invention provides a method for driving a third semiconductor light-emitting device. The third semiconductor light-emitting device comprises a first semiconductor layer of a first conductivity type formed on a substrate and a second semiconductor layer of a second conductivity type formed on the first semiconductor layer. The light-emitting device also comprises an active layer, formed between the first semiconductor layer and the second semiconductor layer, for generating emission light. The light-emitting device further comprises a first electrode for supplying a drive current to the first semiconductor layer and a second electrode, having the shape of a stripe, for supplying a drive current to the second semiconductor layer. The light-emitting device is adapted that the first electrode or the second electrode is a divided electrode divided in the longitudinal direction of the second electrode, and the divided electrode comprises a first electrode portion located on the side of an emitting facet and a second electrode portion located on the side of a reflecting facet. The method comprises the step of applying drive currents, different in magnitude from each other, to the first electrode portion and the second electrode portion so as to induce a self-pulsation.
According to the method for driving the third semiconductor light-emitting device, drive currents, different in magnitude from each other, are applied to the first electrode portion and the second electrode portion of the divided electrode so as to induce a self-pulsation. This allows the relative intensity of noise to be reduced without superimposing a high-frequency signal on the drive currents, thereby making it possible to simplify the drive circuit of the laser device.
In the method for driving the third semiconductor light-emitting device, it is preferable that no drive current is applied to any one of the first electrode portion and second electrode portion for self-pulsation.
The present invention provides a method for driving a fourth semiconductor light-emitting device. The third semiconductor light-emitting device is employed for reading information stored on a storage medium using a reflected beam of laser light emitted from the semiconductor light-emitting device having a lasing cavity. The method comprises the step of injecting a drive current nonuniformly into the cavity to read the stored information.
According to the method for driving the fourth semiconductor light-emitting device, a current can be injected nonuniformly into the active layer, thereby forming an optical absorption region in the active layer. This causes the lasing current threshold to increase and the differential gain of lasing to thereby increase, thus making it possible to reduce the relative intensity of noise for the operation at low output powers.
In the method for driving the fourth semiconductor light-emitting device, it is preferable that the semiconductor light-emitting device is self-pulsation type.
In the method for driving the fourth semiconductor light-emitting device, it is also preferable that the drive current is a high-frequency current.
In this case, it is preferable that the high-frequency current has a frequency of approximately 100 MHz or more.
Incidentally, among the prior-art semiconductor light-emitting devices, available is a laser device, having a divided electrode, like an integrated laser device such as a DBR (Distributed Bragg Reflector) laser device. However, the devices each corresponding to the divided electrodes have functions different from each other, and the crystal structure or the like in the lasing region is different from electrode to electrode.
Furthermore, to apply modulating currents to the prior-art laser device having divided electrodes, the currents applied are different from each other in magnitude, frequency, phase or the like. Furthermore, in the known laser structure having n-side electrodes formed on both sides of the p-side electrode, the n-side electrodes have the same length in the direction of lasing.